The most important results came from the thermal
ionization experiments. The thermal ionization mass spectrometer used in this
work is similar to others described previously (3). It has a single
magnet with 90° deflection and a 30-cm central radius of curvature. It is
equipped with a pulse-counting detection system to allow complete isotopic analyses
to be made on small quantities(<1 ng) of suitable elements ionized from a single filament. The filaments, made of
V-shaped rhenium foil 0.64 cm long and 0.08 cm deep (4),
were baked out at 2000°C before loading the zircons. Ions are formed by
resistive heating of the filament; typical temperatures for this work were
1400° to 1470°C (uncorrected pyrometer readings).

Previous work done to develop a technique for
analyzing small lead samples led to the use of silica gel to enhance ionization
efficiency (5). Because individual zircons are chemically somewhat
similar to silica, we decided to try to analyze lead from individual zircons
loaded directly on the rhenium filament. Such a technique would have several
advantages over traditional methods: contamination would be essentially
eliminated because no chemical separation would be required and, since the
zircons are small (~ 50 μm in diameter), they would provide an approximate
point source of ions, which is known to optimize ion-optical conditions in
the mass spectrometer (6).

Test experiments with zircons from other
localities (7) were uniformly successful; ion signals were observed at
masses (m) 206, 207, and 208 which could definitely be ascribed to Pb
isotopes. To help ensure that we were at the correct ion lens conditions, we
focused on the 138BaO+ peak (the zircons contained some
Ba), which was reasonably intense at 1200°C. Surficial residues left on the
zircons after the acetone wash burned off before the operating temperature of
1450°C, where the lead signal was measured. Great care had to be exercised to
avoid making the temperature too high; very rapid evaporation of the lead
occurred only a little above the operating temperature. Typical count rates
were 100 to 3000 counts per second for 206Pb+. Traces
of thallium (m = 203 and 205) were sometimes observed, but burned out more
rapidly than the lead. Other than thallium, lead gave the only substantive
peaks in the range m = 202 to 210. There was, however, a general background
generated by the sample; chemically unseparated samples such as these zircons
almost always yield such backgrounds. This background has little effect on
the 206, 207, and 208 peaks, but made precise measurement of the 204Pb
signal, which was very small, impossible. For example, in an analysis typical
of these experiments, 1.6 × 105 counts from 206Pb were
collected; the background correction was about 40 counts and, after
correction, 18 counts remained at mass 204. Although these counts are listed
as 204Pb counts in Table 2, more work is needed to determine how
much may be uncompensated background.

Table 2 shows the results of our mass analyses of
filaments loaded with single and multiple zircons from five granite cores.
The range of 206Pb/208Pb values reflects the fact that
this ratio varied from one group of zircons to another, and sometimes varied
during measurements on a single zircon. These variations are not surprising
in view of the ion microprobe analyses, which showed significant U/Th
variations at different points on a single zircon (232Th decays to
208Pb and 238U decays to 206Pb). These
variable 206Pb/208Pb ratios do not furnish any direct
information on differential Pb retention in these zircons. For that purpose,
it is generally accepted that the Radiogenic 206Pb/207Pb
ratios derived from 238U/235U decay are more specific.
We note that Zartman's (8) isotopic measurements of Pb, which was
chemically extracted from zircons taken from the GT-2 core at 2900 m, yield
an adjusted 206Pb/207Pb ratio (9) that
approximates our ratios.

In a conventional chemical extraction of lead
from zircons, the lead measured in the mass analysis is considered to be a
combination of radiogenic lead (from U and Th decay) and nonradiogenic lead
(from common lead contamination and from some initial lead in the zircon).
The radiogenic component is obtained by subtracting out a nonradiogenic
component proportional to the amount of 204Pb. In our experiments,
however, the direct loading procedure virtually eliminated the common lead
contamination, and we circumvented the need to make adjustments for initial
lead in the zircons by accepting only analyses (10) showing a ratio of
204Pb to total Pb of less than 2 × 10−3. Thus the 206Pb/207Pb
ratios shown in Table 2 represent highly radiogenic lead and hence are
potential indicators of Pb retention.

We consider that the most important
observations on the data in Table 2 are: (i) the fact that the 206Pb/207Pb
ratios on single zircons closely approximate the ratio obtained when a group
of similar zircons was loaded simultaneously on a single filament, (ii) the
relative uniformity of the 206Pb/207Pb ratios for
zircons from all depths, and (iii) the fact that the total number of Pb
counts per zircon (the counts in column 4 of Table 2 divided by the product
of columns 2 and 3) shows no systematic decrease with depth, as would be
expected if differential Pb loss had occurred at higher temperatures. Taken
together, items (ii) and (iii) provide strong evidence for high Pb retention
in zircons even for a prolonged period in an environment at an elevated
temperature. These results have possible implications for long-term nuclear
waste disposal.

Zircon
depth (m)

Filamentsanalyzed

Averagezirconsperfilament

Total Pbcounts

Countsof 204Pb

204Pb/
total Pb

Average206Pb/208Pb

Range206Pb/208Pb

960

4

~ 10

1.2 × 106

235

2 × 10−4

9.6 ± 0.3

6.5-9.2

960

4

1

1.3 × 105

35

2.7 × 10−4

9.9 ± 0.4

5.8-14

2170

3

~ 5

8.9 × 105

269

3 × 10−4

10.0 ± 0.4

6.4-12.4

2900

3

~ 4

4.1 × 105

114

2.8 × 10−4

11.2 ± 0.3

4-11.4

3930

2

~ 10

6.5 × 105

132

2 × 10−4

11.0 ± 0.4

5.9-8.7

3930

2

1

8.0 × 104

46

5.8 × 10−4

10.4 ± 0.1

3.1-6.9

4310

7

~ 10

5.6 × 104

1400

2.5 × 10−4

9.7 ± 0.6

3.4-9.8

4310

2

1

1.6 × 105

100

6 × 10−4

9.8 ± 0.4

4.5-10.7

Table 2. Results of thermal ionization
mass measurements for zircons with a 204Pb/total Pb ratio of
less than 2 × 10−3. The background correction was taken from the
208.5 mass position; it was applied to the raw data to obtain the isotopic abundances,
which were used to compute the isotopic ratios. Standard deviations are
listed with the Pb isotopic ratios.